Self-moderation, the killfile, is a feature that every social system needs but is usually worse-implemented than usenet -- which is weird, because usenet does it client-side.

Two forms of moderation exist on usenet. The well-known one is the 'moderated group', where every post passes through to a human gateway before being allowed to post. It's a lot of work, and restricts traffic significantly. It works very well at a high cost and low throughput.

The other one is the anti-spam mechanism, which uses public key infrastructure to sign forged cancel messages which chase the spam around the distribution network. It generally did not work well.